Petroleum distillates such as diesel oil or fuel oil contain, depending on their sources and distillation conditions, normal paraffins which in certain latitudes in winter precipitate as waxy crystals at low temperatures. As the temperature in the oil fails, the crystals tend to grow into large platelets and their presence eventually increases the viscosity of the fuel to an extent that it can no longer be poured (pour point). In the 1960's, ethylene/vinyl acetate copolymers (EVA), such as described in U.S. Pat. No. 3,093,623 and U.S. Pat. No. 3,048,479 were used commercially as additives for lowering the pour point of the oils at fairly low temperatures. The pour point is however some degrees below the temperature at which the wax crystals begin to appear (known as the cloud point) and so the fuel will still pour despite the presence of some wax crystals.
It was also found that depression of the pour point was not the only decisive factor for the winter time handling of paraffin-containing petroleum distillates. Specifically, it was found that at temperatures between the cloud point and the pour point, large wax crystals could sometimes form which could prevent the oil passing through filters despite its ability to pour. To measure this filter plugging tendency, the Cold Filter Plugging Point (CFPP) test was developed and became a European standard. It was found that petroleum middle distillates could more readily be conveyed through pumps and filters, with less danger of blockages occurring due to precipitates of waxy paraffin crystals, by use of suitable additives to hold the crystal growth within such limits that only small crystals precipitated. Such suitable additives were also based on ethylene/vinyl acetate copolymers. These show the surprising effect of not only depressing the pour point, but also substantially restricting the growth in size of the paraffin crystals. This is due to the EVA having a low degree of branching of 6 or fewer methyl side chains, thereby giving an improvement in the CFPP performance of the oil.
However, it has now been found that, in certain fuels, ethylene-vinyl acetate copolymers give unacceptable variation in CFPP results due to randomly occurring aspiration anomalies in carrying out the test. Thus, the refiner may not be able to reliably treat the fuel or may have to treat the fuel with higher additive concentrations than should be necessary.
More specifically, fuel oil compositions being middle distillate petroleum-based fuel oils treated with ethylene-vinyl acetate flow improvers are typically produced to meet a specified target for CFPP performance (referred to herein as the `CFPP Specification`), which is determined by measurement in the CFPP test. Aspiration anomalies give rise to variable results in the test, making the task of accurately determining CFPP performance difficult and giving rise to doubts over whether the fuel oil composition meets its CFPP Specification. These aspiration anomalies have been observed particularly when untreated fuel oils having the following characteristics are used as the base fuel in such compositions;
a wax content of less than 2 wt % at 10.degree. C. below wax appearance temperature, and either PA1 (i) a final boiling point of greater than 355.degree. C.; and either a (90-20%) distillation range of greater than 115.degree. C. or a (FBP-90%) distillation range of 30.degree. C. or greater, or PA1 (ii) a final boiling point of greater than 360.degree. C.; and either a (90-20%) distillation range of greater than 110.degree. C. or a (FBP-90%) distillation range of 25.degree. C. or greater, or PA1 (iii) a final boiling point of greater than 370.degree. C.; and either a (90-20%) distillation range of greater than 100.degree. C. or a (FBP-90%) distillation range of 25.degree. C. or greater, PA1 (A) a major proportion of a middle distillate petroleum-based fuel oil having a wax content of less than 2 wt % at 10.degree. C. below the wax appearance temperature of the fuel, and either PA1 (B) a minor proportion of an additive comprising
all temperatures being measured in accordance with ASTM D-86.
It has been found that this aspiration problem may be overcome by the addition of higher levels of the ethylene-vinyl acetate copolymers in these critical base fuel oils, to the point where the incidence of aspirations becomes negligible. However, this results in the use of excessive amounts of the copolymers and does not represent an economical way of obtaining fuel oil compositions reliably meeting their CFPP Specification.
It has now been discovered that treating the above-defined critical base fuel oils with certain other ethylene vinyl ester copolymer flow improvers leads to specific fuel oil compositions which are less prone to aspirations and which have comparable or even improved CFPP potency. As a result, the fuel producer may produce fuels reliably meeting the required CFPP Specification whilst using substantially less flow improver, so obtaining the desired fuel oil compositions more economically and consistently.